1. Field of the Invention
This invention relates to video processing apparatus operable with a first video means for first series of video images referenced by means of first timecodes and a second video means for a second series of video images referenced by second timecodes. In particular, the invention relates to video processing apparatus which, in order to process video images associated with the first and second video means, includes control means for synchronising the operation of the first and second video means.
2. Description of the Prior Art
Professional video processing is typically performed using timecodes such that each frame (and/or field) of video in a video sequence is uniquely identified by a timecode. A timecode is typically defined in terms of hours: minutes: seconds: and frames. Optionally, the fields in interlaced video may be uniquely identified as well. Conventional professional video players and recorders can be instructed to proceed to a frame identified by a particular timecode and then to playback or to record from that frame in the sequence. When performing an edit, the video apparatus can be instructed to start an edit at a first timecode and to terminate the edit at a second timecode.
Professional video studios are typically provided with a standard reference source identifying a standard reference frequency for that studio. Typically the studio reference will either be 50 Hz, or 60 Hz. By supplying this reference to all the video units in the studio, the operation of those units can easily be synchronized. Thus, for example, where it is desired to perform an edit between a source video tape recorder (VTR) and a destination VTR using processing apparatus, the operation of the three units can be synchronised using the common studio reference.
If, for example, it is intended to use a sequence of video images on a tape in a source VTR to edit part of a sequence of video material on a tape in a destination VTR, this can be performed in the following manner. Each video frame and field on the tape is identified by means of timecodes. Accordingly, an edit controller can issue a command to a first one of the VTRs (eg. the source VTR) to go to a timecode "S1-d1 fields" and to switch to a play mode based on the standard studio reference. "S1" is the start timecode for the edit sequence on the first VTR and "d1" is a number of timecodes preceding the start timecode which allows the first VTR to lock to the reference and to synchronise the second VTR before reaching the start timecode S1.
The edit controller instructs the second (e.g. the destination) VTR to go to a timecode "S2-d2 fields" and to enter the play mode. The destination VTR spools to the point identified and starts to run based on the common studio reference. "S2" is the start timecode for the edit sequence on the second VTR and "d2" is a number of timecodes preceding the start timecode which allows the second VTR to get up to normal play speed and to be synchronised to the first VTR. In order to synchronise the running of the second VTR to that of the first VTR so that the fields of video information are in the correct relationship, the edit controller reads the timecodes from the first and second VTRs and computes the error between the timecodes. On the basis of the error signals, the edit controller sends commands to one of the VTRs to vary its speed until the timecodes are found to be in the correct relationship. After this the edit operation can be performed when the start timecodes have been reached.
The above process is relatively straightforward and is a standard part of conventional video processing. However, it assumes that both the source and destination VTR are operating in response to a common timing reference, so that the timing reference for the source and destination VTRs are in phase and accordingly that the timecodes for each frame and field of video information on the tapes of the source and destination VTRs are in a predetermined relationship as well.
However, situations exist where a source video device and a destination video device may not be operating in response to a common standard timing reference. One example of such a situation is where video standards conversion is effected between a source video device and a destination video device. When converting between a sequence of source video images at, fop example 60 Hz, and a sequence of video images at, for example 50 Hz, it is necessary to process the video signals (e.g. to effect motion compensation) in order to provide a high quality output. Many or the processes which can be performed can be adjusted in order to take account of the content of the source video images.
The operator of the video standards conversion equipment can therefore tailor the processing to be performed on the source video image in order to give optimum output video images. It is not an easy matter to tailor the processing of the input video images in such a manner that artifacts and undesired effects are eliminated from the output video sequence. Undesired effects can result from the movement of objects in the source video sequence. Due to the need to convert between video having 60 images per second to video having 50 images per second, the movement in the converted images may no longer be smooth or natural. For example, with one motion compensation algorithm, the stripes on the shirt of a person moving in the source video sequence may swim in the output video sequence. By applying a different processing algorithm to the images, it may well be possible to eliminate or mitigate this undesired effect.
However, to date, where such undesired effects have been observed in a video sequence output by a video standards converter, it has been necessary to reprocess the complete sequence of video images using modified video processing in order to avoid undesired effects. This is because it has not been possible to edit a section only of the video sequence which manifests the undesired effects due to the difficulty of accurately synchronising the operation of the source and destination video tape recorders to enable the edited portion to be inserted cleanly. As the source and destination video devices are driven at different frequencies in response to respective timing references, the phase relationship between the timecodes and the references varies from image to image within the video sequences. Having to reprocess the complete video sequence is an expensive and time consuming process. Also, although changing the video processing performed on the sequence may cure artifacts at one point in the sequence, the use of other processing operations can cause other artifacts to appear at other points in the sequence.